1. Field of the Invention
This invention relates primarily to the fabrication and structure of an electrical winding in which individual wires must be maintained motionless, and more specifically, this invention relates to a method of fabricating and the structure of a superconductive electrical winding formed in the slots of a dynamoelectric machine rotor.
2. Description of the Prior Art
In rotating machine applications, a superconductive wire utilized for the windings is normally in the form of a multiplicity of relatively small filaments of superconductive material. These filaments are then encased in a copper matrix to form a superconductive wire. At cryogenic temperatures (approaching absolute zero), the superconductive material is resistanceless and hence, at these temperatures, the copper appears as an insulator.
While the above-identified wires have extremely high current carrying capacity at cryogenic temperatures, it is still necessary to obtain current capacities many times larger than those found in a conventional wire of this nature for many types of electromagnetic devices. Such large current carrying capacities are necessary because of the design criteria of machines with extremely large power capabilities.
In order to achieve the requisite high currents, it is necessary to either make the wire larger or to fabricate a cable from many wires. Making the wire larger results in many detrimental effects with respect to the properties of the superconductive wires. Therefore, cabling a number of wires provides more beneficial results while providing the higher current capacities that are required.
While cabling has many advantages in the production of greater current capacities, such as permitting the transposition of wires to achieve the resultant benefits, there are still significant problems that have been left unsolved by prior art approaches. Prior art cables, both braided and transposed, have usually been made either from fully insulated or uninsulated wires potted in solder (usually a silver-tin eutectic).
When a cable is made up from fully insulated wires, it has the benefits of low losses in an applied AC field, such as that due to negative sequence currents. In addition, the porosity of this type of structure permits the passing of a coolant (a cryogenic refrigerant in the case of superconductive windings) therethrough. However, this structure has the disadvantage that the wires may move relative to each other, resulting in mechanical instability. Of course, in a superconductive coil the movement of any wire relative to the magnetic field can result in "quenching" or loss of the superconductive effect in the entire coil.
In cables made from wires potted in solder, mechanical stability of the coil is achieved. However, large eddy currents flow in the presence of an AC field, with the resultant high losses. Attempts have been made to pot the wires in a material that would provide some insulation, such as an epoxy bonding material, but when used in these ways it was found that the epoxy did not provide sufficient mechanical strength as a potting compound. In addition, the epoxy was found to be inadequate as an insulating material. Thus, the present assumption of those skilled in this art is that it is necessary to live with either the mechanical instability of cables made from insulated wires or the high losses resulting from uninsulated wires potted in solder.
In addition to its use as a potting agent, bonding material has been coated on insulated wire. Again, however, the mechanical strength of such bonding material standing above is not sufficiently great to preclude relative movement between the wires of a winding.
Large motor and generator rotor windings with large bars for conductors have been compressed by the use of a pressurizing medium located in the rotor slots. This procedure is utilized to compact the bars against the slot wedges and to preclude vertical movement in the slots. However, this technique is limited to structures involving a relatively few large bars in the slots where the prime desire is to eliminate vertical motion. To use such an approach with structures in which a relatively large number of wires are located in a slot would result in damage to the wires, upon application of the large pressures needed to compress the wires sufficiently to preclude both vertical and horizontal movement. In addition, compacting the wires to this degree would completely eliminate the interstices useful for cooling purposes.